A polyolefin is a polymer produced from a simple olefin as a monomer. For example, polyethylene is the polyolefin produced by polymerizing the olefin ethylene. An olefin is also called an alkene with the general formula CnH2n and a polyolefin is also known as a polyalkene. Polypropylene is another common polyolefin which is made from the olefin propylene.
By definition, cross-links are connecting points that link one polymer chain to another. There are two types of crosslinks: chemical crosslinks are covalent bonds that bridge the polymer chains; and physical crosslinks are hydrogen bonds or chain entanglements between neighboring chains.
There are two commonly employed processes for the formation of chemical crosslinks in polymers. These are deficient in that they either change the chemical composition of the polymer or create short polymer chains.
The first such commonly employed process involves use of a crosslinking agent, such as peroxide, vinylsilane, or ethylene glycol dimethacrylate. The crosslinking agent is pre-mixed with un-polymerized or partially polymerized resin powders. Cross-links are then formed in the resultant material by chemical reactions that are initiated by heat, pressure, change in pH, or radiation during the consolidation step. The main advantage of this method of crosslinking is that the distribution of crosslinks is substantially uniform. The major drawback of this method is that the resultant material often has a different chemical composition than that of the starting material due to the inclusion of the crosslinking agent. For medical implant applications, for instance, a pure polymer with proven biocompatibility is preferred.
The second method of crosslinking exposes a polymer to a radiation source, such as electron beam or gamma rays. During irradiation, free radicals are produced by breakage of covalent bonds in the polymer. Free radicals then react with each other during irradiation or post-irradiation to form crosslinks.
The main advantages of this second method are process simplicity, low cost, and no change in chemical composition. There are major shortcomings for the second method, however. The first major shortcoming is that the distribution of crosslinks in the polymer is non-uniform. The non-uniformity arises from the radiation dose distribution in the polymer.
The second major shortcoming is that irradiation penetration from the surface of the polymer may not be thorough. For e-beam irradiation, the limited power of penetration results in a higher dose near the surface while lower or zero dose in the interior. Although gamma rays penetration is much deeper than e-beam irradiation, larger blocks of material (such as 4 inches or thicker) still show dose variation between a surface zone and the interior. Independent of block size, radiation dose goes through to a maximum point at a sub-surface depth (about 2 to 7 mm depth, depending on the material density, radiation conditions, etc.) before its decline into the interior. This phenomenon is known as secondary ion or Compton Effect.
The third major shortcoming is that crosslinking reactions of free radicals are often incomplete; leaving residual free radicals in the material that can cause oxidation or other chemical changes during storage or field applications. It is well known that material properties are deteriorated when a polymeric material is oxidized.
The fourth major shortcoming is that during high energy radiation, short chains are formed as a result of chain breakage along the backbone of the polymer. In general, the amount of short chains is increased with increasing radiation dose. Short chain formation tends to weaken the material strength as the effective molecular weight is reduced.
As defined earlier, physical crosslinks do not involve covalent bonds but enhance molecular network through hydrogen bonding or chain entanglements. In crystalline polymers, chain entanglements exist in the amorphous regions which are critical micro-structures for material strength.